Allow more options to register with signal

[nicobo]

Currently it is only possible to link with an existing account (which makes sense if you don't want to lock a phone number with the bot).
However signal-cli allows more ways to register, including SMS and voice verification => allow to register with those methods.

This should probably start with allowing to pass signal-cli options "as-is" to the bots (e.g. --signal-cli-options "-a +1234567")...